Wind tunnels are used to conduct aerodynamic and aeroacoustic tests of scale models of various types of vehicles and particularly of aircraft. Aerodynamic tests traditionally use a closed section configuration because it is a mature technique in which the air flow impinging on the mock-up is not really affected by the walls and because the associated corrections are very well known. However, the measurements aeroacoustic are usually performed in an open section because reverberations in the walls of the tunnel are thus avoided. This means that the aerodynamic and aeroacoustic tests are done separately, with the subsequent duplication of efforts and costs.
Even though various materials capable of absorbing sound are well known in the prior art, no specific proposal, however, providing a coating of a closed section of a wind tunnel with a high degree of acoustic absorption which allows easily taking the measurements which are required in aeroacoustic tests of, particularly, aircraft models, is known.
The commercial solutions for the absorption of sound using porous and fibrous materials are not applicable for said coating due to the fact that the air jet circulating inside the tunnel at a high speed would end up carrying off said materials, with the subsequent loss of the acoustic absorption properties.
In relation to microperforated panels (MMPs) which, in theory can be considered applicable to said coating, several proposals which have been used in several industrial sectors are known. The MPPs proposed by Maa (D. Y. Maa, (1997), “Potential of microperforated panel absorber” J. Acoust. Soc. Am., 104, 2861-2866) provide absorption of the sound by visco-thermal losses in sub-millimetric perforations made on a panel and therefore do not require the addition of fibrous materials. In order to tune the absorption in the frequency band of interest, it is necessary to have these MPPs in front of a rigid wall, leaving an air cavity having a certain thickness.
The use of MPPs for the absorption of the sound in various environments is well known in the art and has been the object of several patents.
U.S. Pat. No. 5,700,527 describes the use of microperforated glass as absorbent materials in the construction of buildings. They are simple glass MPPs having a thickness t in the range of 0.2≦t≦30 mm, with circular perforations having a diameter d in the range of 0.1≦d≦2 mm and air cavities having a thickness D in the range of 20≦D≦500 mm.
ES 2 211 586 describes the use of MPPs for coatings in means of transportation, such as land vehicles, trains, ships and airplanes, with panels having a thickness t in the range of 0.2≦t≦5 mm, perforations having a diameter d in the range of 0.05≦d≦2 mm, and perforation percentages p in the range of 0.2≦p≦4%. The air cavity in this case can be filled with spongy material or wadding.
EP 1 382 031 describes the use of multilayer MPPs for absorbent coatings engine exhaust systems or in turbines. The metal have thicknesses of t<0.2 mm, perforations having diameters of d<1 mm, and perforation percentages of p<1%.
U.S. Pat. No. 6,675,551 describes low-cost thick MPPs for their application as constructive elements. The panels can be made of wood, synthetic material or laminated gypsum, and can be combined with other absorbent materials, such as foams, mineral wools or acoustic fabrics. The panels can have thicknesses t in the range of 6≦t≦30 mm, with circular perforations having diameters d≦2 mm, and perforation percentages of p≦4%.
U.S. Pat. No. 6,617,002 describes MPPs using polymeric films. Since these films have a rigidity of less than 107 dynes/cm, with a thickness of t<0.38 mm, the model includes its elastic properties. Another novelty of this patent is the use of conical perforations, with a major diameter of d1<0.5 mm, and a minor diameter of d2<0.15 mm.
As can be inferred from the foregoing, the known proposals are aimed at solving specific acoustic insulation problems very different from those of an anechoic coating of a wind tunnel.
However, it is desirable to have such coating and the present invention aims to solve this need.